The present invention relates to an electric discharge machine to accurately index the rotation angle of a spindle to which a tool electrode is attached. More particularly, the present invention relates to a locking device for locking a spindle of an electric discharge machine.
In many cases electric discharge machines are used to shape a workpiece into a high precision mold by generating electric discharges between a tool electrode and the workpiece. Generally, a spindle to which the tool electrode is attached moves in the direction of a Z-axis, while a table to which the workpiece is fixed moves in a plane defined by an X- and Y-axis. Electric discharge machines that can accurately specify the rotation position of a spindle about the Z-axis is well known. With this type of electric discharge machine, a rotary encoder having a high resolution of, for example, 360,000 divisions, is attached to a servo motor which causes the spindle to rotate. Being able to index the rotational angle of the spindle in this way is known as C-axis control. In order to lock the indexable spindle, a locking device equipped with a brake disk which produces a strong braking force has conventionally been used.
A conventional locking device for locking an indexable spindle will now be described with reference to FIG. 5.
A spindle 1 having a tool electrode held at its lower end is contained inside a housing 3. A thin brake disk 51 having a slight flexibility at an outer edge is fixed to the spindle 1. A fixed frame 52, in which a few brake shoes 54 are spaced equidistantly about the spindle 1, is fixed to the housing 3. A number of cylinder members 53 forming cylinder chambers 53a are fixed to the frame 52. Pistons 55 and springs 56 for resiliently retracting the pistons are respectively housed in the cylinder chambers 53a. Brake shoes 55a are attached to tips of the pistons 55 so as to face the brake shoes 54. Connecting pipes 57 leading to the cylinder chambers 53a are attached to the cylinder members 53. As shown on the right hand side of FIG. 5, if compressed air is supplied to a cylinder chamber 53a through a connecting pipe 57, the piston 55 is advanced and the brake disk 51 is pressed against the brake shoe 54 by the brake shoe 55a. As a result, the brake disk 51 is locked between the brake shoes 55a and 54. At this time, the outer edge of the brake disk 51 bends downwards. As shown on the left hand side of FIG. 5, if the supply of compressed air is stopped the piston 55 retreats, and very small space is formed between the brake shoes 55a and 54 and the brake disk 51.
In this way, when the brake disk 51 is locked between the brake shoes 55a and 54, the outer edge of the brake disk 51 is bent. If this deformation is carried out repeatedly the outer edge of the brake disk 51 becomes set in the deformed state so that it undulates unevenly. As a result of this, there is force in a direction causing rotation of the brake disk 51 derived from pressing against the brake disk 51. The spindle 1 then becomes locked at an incorrect rotation angle due to this component of force.
An object of the present invention is to provide a locking device for accurately locking a spindle at a desired, rotational angle, even if it has been used over a long period of time.
Additional objects, advantages and novel features of the invention will be set forth in the description that follows, and will become apparent to those skilled in the art upon reading this description or practicing the invention. The objects and advantages of the invention may be realized and attained by practicing the invention as recited in the appended claims.
According to the present invention, there is provided a locking device for an indexable spindle to which a tool electrode is attached, comprising a brake disk fixed to the spindle, an upper brake member movable in the axial direction of the spindle between a first position and a second position closer to the brake disk than the first position, a lower brake member movable in the axial direction of the spindle between a third position and a fourth position closer to the brake disk than the third position, an upper disk arranged facing upper surface of the brake disk across a minimum gap when the upper brake member is at the first position, a lower disk arranged facing lower surface of the brake disk across a minimum gap when the lower brake member is at the third position, and drive means for moving the upper and lower brake members in the axial direction of the spindle. When the upper brake member is at the second position and the lower r brake member is at the fourth position, the upper and lower brake members press the upper and lower disks against the brake disk to lock the spindle.